As one of the methods for increasing the recording density of a magnetic recording medium, a perpendicular magnetic recording medium attracts attention. As the magnetic layer of a perpendicular magnetic recording medium, various ferromagnetic metal films, not only films such as a cobalt-chromium alloy thin film, but also cobalt-vanadium, cobalt-ruthenium and cobalt-oxygen films have been investigated and high density recording has been achieved. But, they still have some drawbacks.
For example, in comparison with a coating type recording medium comprising a magnetic recording layer containing a binder, the perpendicular recording magnetic media have low mechanical strength so that they are apt to be damaged by the sliding contact against a magnetic head.
When the ferromagnetic metal film is formed on a flexible non-magnetic substrate such as a polymeric film, the flexibility of the substrate is considerably deteriorated so that it becomes difficult to maintain stable contact between the magnetic head and the surface of the recording medium.
Furthermore, since the magnetic recording medium comprising a non-binder type metal film has good electrical conductivity due to the metal recording film, eddy current is generated by the sliding contact of the medium against the magnetic head, which adversely affects recording and reproducing characteristics.
For production of a conventional horizontal magnetic recording medium, Japanese Patent Publication No. 3137/1982 discloses the following method:
In this method, a magnetic recording medium is produced by means of a sputtering apparatus as shown in FIG. 1. The sputtering apparatus comprises vacuum chamber 21, in which anode 22 and cathodes 23a and 23b are installed in place and connected with a high voltage source (not shown). Vacuum chamber 21 is evacuated through vacuum outlet 24, and then supplied with an inert gas such as argon gas from needle valve 25.
To anode 22, disc form non-magnetic substrate 26 is attached with facing its surface to cathodes 23a and 23b and rotated at a predetermined rate by means of motor 27. On cathodes 23a and 23b, cobalt target 28 and polytetrafluoroethylene target 29 are placed, respectively.
After reducing the pressure of vacuum chamber 21 to high vacuum by evacuating it through outlet 24, argon gas is introduced in the chamber through needle valve 25. Then, when non-magnetic substrate 26 is rotated by motor 27, the substrate is sputtered simultaneously with cobalt and polytetrafluoroethylene from targets 28 and 29, respectively, to form a composite film of cobalt and polytetrafluoroethylene on the substrate. During sputtering, the voltage applied to each of the cathodes 23a and 23b is independently controlled.
This method, however, still has some drawbacks. That is, since substrate 26 is rotated during sputtering, only cobalt is accumulated on a part of the substrate, and that being when said part faces cobalt target 28, while only polytetrafluoroethylene is accumulated when substrate 26 is rotated and faces polytetrafluoroethylene target 29. As a result, thin layers of cobalt and polytetrafluoroethylene are alternately formed.
A structure in which the thin layers of the magnetic material and an organic polymer are alternately laminated is suitable for the production of the horizontal magnetic recording medium but not for the production of the perpendicular magnetic recording medium having satisfactory magnetic characteristics for high density recording.
IBM Technical Disclosure Bulletin, Vol. 25, No. 9, February 1983 describes a method for producing a perpendicular magnetic recording medium as follows:
Said method is illustrated by making reference to FIG. 2. Firstly, on non-magnetic substrate 31, ferromagnetic metal layer 32 with a specific thickness is formed. Then, a mask 34 having windows 33 is placed on metal layer 32 and the metal layer is perpendicularly plasma etched so that parts of the metal layer 32 corresponding to windows 33 of the mask are etched to form bores 35 (see the right half of FIG. 2). Thereafter, bores 35 are filled with, for example, polyimide resin 36 to form a magnetic layer consisting of a composite material of the ferromagnetic metal and the organic polymer (see the left half of FIG. 2).
In the above method, since bores 35 of metal layer 32 should be filled with organic polymer 36, the diameter of the bores should be made large so that the volume ratio of the organic polymer in the magnetic layer inevitably increases to about 80% by volume, whereby durability of the magnetic layer decreases. Further, due to plasma etching and organic polymer filling, the production process becomes complicated and the apparatus becomes large resulting in an increase in the production cost of the magnetic medium.